Microporous membranes are useful as battery separator film (“BSF”) for primary and secondary batteries. These batteries include lithium ion secondary batteries, lithium ion polymer secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, etc.
Microporous polymeric membranes can be used as battery separator film (“BSF”) in, e.g., lithium ion batteries. Lithium ion batteries in which the electrolyte is a gel electrolyte or polymeric electrolyte, e.g., an electrolyte that is contained within a polymeric medium, (“lithium ion polymer batteries”) generally utilize BSFs comprising a polymer compatible with (e.g., has affinity for) the polymeric medium in which the electrolyte is contained. BSFs for lithium ion polymer batteries generally have a significantly smaller thickness compared to BSFs commonly used in, e.g., cylindrical and prismatic-format lithium ion batteries.
BSFs comprising polymer have increased polymer mobility at elevated battery temperature, which leads to a significant permeability decrease. This effect is beneficial in BSFs because the permeability decrease at elevated temperature results in a decrease in battery electrochemical activity, thereby lessening the risk of battery failure under overcharge, rapid-discharge, or other high-temperature battery conditions. Since battery internal temperature can continue to increase even at reduced electrochemical activity (e.g., from temperature overshoot), it is desirable to increase membrane thermal stability at elevated temperature to further lessen the risk of battery failure. This can be accomplished by including a high melting-point species (e.g., polypropylene) in the membrane's polymer to increase the BSFs meltdown temperature to a value ≧145° C. The temperature difference between polyethylene and polypropylene melting points and their physical incompatibility leads to difficulties in producing membranes containing both polymers, particularly when the membrane is thin, as is the case in lithium ion polymer batteries.
Another important BSF property is the BSFs resistance to pin puncture (commonly referred to as the BSFs “pin puncture strength”). Since BSF pin puncture strength is proportional to BSF thickness for BSFs of substantially the same composition and porosity, batteries utilizing thin BSFs such as lithium ion polymer batteries benefit when the BSF has a relatively high pin puncture strength per unit thickness. It is desired, therefore, to produce a relatively thin, high strength polymeric membrane having a relatively high meltdown temperature.